Procedure for obtaining rigid polyurethane foams.

Process for preparing rigid polyurethane foams by reacting a) polyisocyanates with b) compounds having at least two hydrogen atoms reactive toward isocyanate groups in the presence of c) blowing agents characterized in that compounds are used together with, at least two hydrogen atoms reactive toward isocyanate groups b) at least one polyol Graft b1) can be obtained by the in-situ polymerization of olefinically unsaturated monomers in a polyether alcohol b1i), also the alcohol of polyether b1i) has a functionality of 2 to 4 and a hydroxyl number in the range of 100 to 250 mg KOH / g, its polyether chain comprises propylene oxide and up to 20% by weight, relative to the polyether alcohol b1i), of ethylene oxide, and olefinically unsaturated monomers as acrylonitrile and styrene are used in a weight ratio of acrylonitrile: styrene of> 1: 1 to 4: 1, and the conversion ll eva out in the presence of at least one compound blendable in the presence of at least one alcohol b1i polyether) with at least one olefinic double bond b1ii).